The invention relates to a method of applying a layer of photosensitive material to a wafer of semiconductor material, in which photosensitive material is applied to the wafer in liquid form and the wafer is then rotated in order to obtain the desired thickness of the layer, photosensitive material present at an edge of the wafer being removed by means of a jet of a liquid directed onto this edge to dissolve the photosensitive material. The invention further relates to an apparatus for carrying out this method and to a semiconductor wafer obtained by means of this method.
In general, a photosensitive (photoresist) layer is applied to a semiconductor wafer for the manufacture of microelectronic circuits by means of machines particularly designed for this purpose. These machines comprise a rotatable horizontally arranged table the speed of which is variable. The semiconductor wafer is placed on this table and is held on it, for example, by means of a vacuum. A metered quantity of liquid photosensitive material is applied to the semiconductor wafer. By rotation of the table, the photosensitive layer is subjected to a centrifugal force and at a given speed of rotation the desired ultimate thickness of the photosensitive layer is attained.
In this process, however, an undesired effect occurs at the edge of the semiconductor wafer; at the edge the vertical part of the wafer is also covered with photosensitive material. This photosensitive material overhanging the edge is liable to break off as it is very brittle. Thus leads to an undesirable contamination both in the transport means and in the further processing apparatus of the semiconductor wafer, whereas an extremely high degree of purity is required in order to obtain a favorable yield.
In order to avoid this undesirable effect, the overhanging photosensitive material is removed by means of a suitable liquid organic solvent, such as, for example, acetone. A jet of the solvent is then directed onto the edge of the rotating semiconductor wafer. Due to spattering, a part of the active surface of the semiconductor wafer is often touched by the solvent, which adversely affects the yield.